The Replicants are organic androids created by the Tyrell company in the year 2019. Replicants are created as slaves and called the name "skin-job" as a mean of belittling them. they are sent to the off-world colony to fight, work in dangerous jobs, kill and even be the human's "basic pleasure model."
Replicants are not allowed back on Earth due to the fear that they will gain to much power. When they do get to Earth, special bounty hunters, called Bladerunners, are sent to "retire" (kill) them. this seems hardly needed, because of a fail safe installed in them which kills them after four years.